This application claims the priority of German patent document 103 23 644.9, filed 26 May 2003, the disclosure of which is expressly incorporated by reference herein.
The invention relates to fuel cells with polymer electrolyte membranes (PEM).
In unhumidified or partially humidified fuel cells of this type, drying of the membrane electrode assembly (MEA) increases in the reaction area at the cathode entry, and the effective power of the cell deteriorates or, in extreme cases, the cell is damaged. The reason for such drying is the relatively dry cathode gas flow, which is also too high per unit area of the active cell, in this region, so that the product water which is formed at the catalyst is insufficient to humidify the MEA in order to reliably prevent drying-out effects. This applies in particular to fuel cells with conventional gas distributor structures (flowfields), in which the width of the gas-carrying passages is constant from the cathode entry to the cathode exit.
To avoid drying-out effects, prior art techniques include the use of flowfields for unhumidified or partially humidified PEM fuel cells made from porous material in order to balance out the steam partial pressure along the cathode passage.
At the cathode entry, the steam partial pressure is low because the intake air stream is unsaturated. This stream is then gradually enriched with water along the cathode passage by the product water which is formed, so that the steam partial pressure rises. Finally, at the cathode exit the steam partial pressure has risen to such an extent that liquid water is often formed through condensation and is taken up by the porous plates. The stream of water is guided in the plate, through diffusion, to the dry cathode entry and used to humidify the incoming cathode air stream in this region of the membrane.
The drawbacks of this arrangement are the high cost of the bipolar plate material comprising a porous structure, the considerable thickness of the bipolar plate and the sensitivity of a system of this type to high cell temperatures (as are desired, for example, in order to reduce the size of the cooling system of the fuel cell).
One object of the present invention is to provide a suitably configured gas distributor structure, so as to match the gas flows within the active cell area of PEM fuel cells, achieving a substantially uniformly distributed humidification state across the membrane area.
This and other objects and advantages are achieved by the gas flow structure according to the invention, in which starting from the cathode entry port, the active gas volumetric flow per unit area on the MEA is locally varied by changing the number and/or the cross sectional area of the gas-carrying passages, so that the flow decreases toward the cathode exit port. Suitable configuration of the passage structure according to the invention allows the steam partial pressure to be optimally matched to the local conditions. The mass conversion is improved in these regions as a result without the electrolyte's drying out. At locations in the flowfield with a low gas humidity, the measures according to the invention (increased number of passages or passage width) significantly reduce the flow velocity. As a result, the transfer of water from the MEA to the gas flow is reduced and drying of the MEA is diminished.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.